142 Tlie America?i Geologist. September, i898 
Quartz is abundant as a secondary mineral in some of the 
altered phases of the gabbro. It is never present in the fresh 
rock. With the feldspar it forms a pegmatitic texture like 
that foimd in the augite syenite of the red rock member de- 
scribed by Irving (14, p. 114). In some sections grains of 
feldspar are entirely replaced by quartz, which then has a typi- 
cal granitic texture. There are all gradations in texture be- 
tween the feldspar and the quartz. The former in some sec- 
tions shows only a few small specks of quartz, which becomes 
more abundant until the feldspar is entirely replaced. The 
quartz has also been formed between crystals of feldspar, in 
which case it replaces some of the other minerals. In some 
sections it is filled with minute inclusions of a dark substance. 
Hematite occurs as small red flakes and as an earthy de- 
composition product in some of highly altered gabbro. When 
the iron bearing minerals are abundant the hematite gives a 
brick red color to the rock. 
Limonite is present as a yellow earthy decomposition 
product. It is noticeable in small quantities in nearly all sec- 
tions which show alteration. 
Leucoxene occurs in small quantities as an alteration 
product of the titaniferous magnetite. It has been noticed as 
a white powder in some of the highly altered magnetite 
masses, and in thin sections it is found as a white border 
around some of the magnetite grains. 
Kaolin is present as decomposition product from the felds- 
par. It is present usually in fine flakes within the feldspar. 
In some parts of the decomposed rock the feldspar is entirely 
altered to kaolin, which then forms a dull white substance 
easily powdered. 
Serpentine is the usual "alteration product of olivine, and is 
abundant in the forellenstein. It occurs as a fibrous sub- 
stance often stained red by iron oxide. 
Calcite occurs in small veins in the gabbro, and is the 
result of alteration in the rock. Some of this mineral may 
be derived from outside sources, but part of it, which is asso- 
ciated with the decomposed rock, is probably derived directly 
from the gabbro. 
Malachite occurs as a green coating upon the rock and 
in fractures in localities where copper and chalcopyrite occur 
in the gabbro. 
